vertex slide within face
You can move a vertex along the normal of the associated face. This doesn't ensure that the face was planar in the first place.


Switch to Normal orientation mode.
Select a Face in face mode.
Scale its Z normal to 0 in face mode. SZZ0


At this point nothing should have changed. If anything changed your face was not planar to begin with.


Make sure all vertices of the face only belong to a single face by ripping them. V
Move them along the face normal. G&#8679; ShiftZZ